Loss of the nuclear lamina protein Otefin causes Checkpoint kinase 2-mediated death in female germline stem cells.

Loss of the nuclear lamina protein Otefin causes Checkpoint kinase 2-mediated death in female germline stem cells. Lacy J. Barton, Pamela K. Geyer. Dept Biochemistry, Univ Iowa, Iowa City, IA.

Tissue homeostasis is supported by adult stem cells. Emerging evidence suggest that compromised adult stem cell function is associated with human diseases caused by mutations in components of the nuclear lamina, such as LEM Domain (LEM-D) proteins. How defects in LEM-D proteins affect stem cell function is unclear. To gain insights into these mechanisms, we study the Drosophila LEM-D protein Otefin (Ote), which is required for female germ cell differentiation and germline stem cell (GSC) maintenance. Our recent studies have shown that progressive loss of GSCs in ote mutants occurs in the absence of the early germline differentiation factor, Bag of marbles, indicating that GSC loss is due to cell death. To understand this process, we have performed a genetic suppression screen, testing candidates known to have a role in cell death. Strikingly, loss of the DNA Damage Response (DDR) transducer Checkpoint kinase 2 (Chk2) fully suppresses ote^-/- GSC loss and permits all stages of oogenesis. Despite normal morphology, ote^-/-, chk2^-/- eggs that are fertilized with wild type sperm fail to undergo even the earliest stages of embryogenesis, suggesting that the critical defect persists. Surprisingly, immunohistochemical analysis of ote^-/-, chk2^-/- germ cells showed normal chromatin organization, capped telomeres and a lack of DNA damage foci. In addition, ote^-/- GSC phenotypes were not suppressed by the loss of other DDR components, including those upstream and downstream of Chk2. Together, these data suggest that DNA damage is not the cause of GSC death in ote mutants. We are currently investigating the initiating cause of Chk2 activation, including defects in transposon silencing, Chk2 sequestration and nuclear lamina integrity. These studies will provide needed insights into mechanism of stem cell dysfunction caused by mutations in LEM-D proteins.